Blood pressure refers to the force exerted by circulating blood on the walls of blood vessels. It, along with your pulse, make up the vital signs. The pressure of the circulating blood decreases as blood moves through arteries, veins, and capillaries; the term blood pressure generally refers to arterial pressure. Using a blood-pressure cuff, or sphygmomanometer, the pressure in an artery of your arm is measured.

Blood pressure is composed of two measurements: diastole and systole. Systolic pressure is the measurement of peak pressure in an artery at the beginning of the cardiac cycle or during ventricular contraction. Diastolic pressure is the measurement of the lowest pressure during the resting phase of the cardiac cycle or during ventricular relaxation. A blood-pressure reading is given by the ratio of systole over diastole.

Blood pressure is measured using a blood pressure cuff with the patient seated. The cuff is inflated by pumping a small rubber bulb until the artery in the arm is completely blocked. Listening with a stethoscope to the artery at the elbow, the pressure in the cuff is slowly released. As the pressure in the cuffs falls, a "whooshing" or pounding sound is heard to indicate that the blood flow has started again in the artery. The pressure at which the whooshing sound begins is the systolic pressure. The cuff pressure is further released until the whooshing sound can no longer be heard, and this is recorded as diastolic blood pressure.

An older method, which is still used today, has the cuff attached to a measuring unit on the wall and the doctor reads the height (mm) of mercury and records this number as the systolic pressure. Newer cuffs have a pressure gauge built in or are digital.

The tables below show the ranges for low, high, and normal blood pressure. An average reading means that, in terms of your circulatory system, you are in relatively good circulatory system health. High or low blood pressures can indicate that some system in your body, possibly the circulatory system, is impaired and unable to maintain homeostasis. In the following activity, you will explore factors that cause high blood pressure, known as hypertension.